The purpose of the present study was to investigate the development and recovery of muscle fatigue in the upper trapezius muscle by analyzing electromyographic signals. Six male subjects performed a simulated typewriting task for four 25-min sessions. During fatigue and the following rest periods, subjective fatigue and surface electromyography (EMG) from the trapezius muscle during isometric contraction at 30% maximum voluntary contraction (MVC) were periodically measured in the interval. We detected a significant decrease in muscle fiber conduction velocity (MFCV) (P = 0.008) and median frequency (MDF) (P = 0.026) as well as an increase in root mean square (RMS) (P = 0.039) and subjective fatigue (P = 0.0004) during the fatigue period. During the recovery period, subjective fatigue decreased drastically and significantly (P = 0.0004), however, the EMG parameters did not recover completely. Thus, physiological muscle fatigue in the trapezius developed in accordance with subjective muscle fatigue during typewriting. On the other hand, differences between the physiological and subjective parameters were found during recovery. Further studies should be necessary to reveal the discrepancy could be a major factor of a transition from temporal phenomena to serious chronic muscle fatigue and to identify the necessity of some guidelines to prevent VDT work-related chronic muscle fatigue in the trapezius.

The effect of bathing on recovery of muscle fatigue in the trapezius was examined in 6 healthy male adults. The subjects performed four sets of 25-min typewriting tasks and then rested on a chair for one hour. The development and recovery of muscle fatigue in the upper trapezius was periodically monitored with an electromyogram (EMG) in parallel with subjective muscle fatigue, surface and core temperatures, and near-infrared spectroscopy tests. The surface EMG was recorded during a 15-sec contraction at 30% maximum voluntary contraction to evaluate the muscle fatigue. During typewriting, muscle fiber conduction velocity and median frequency significantly decreased in parallel with an increase in subjective muscle fatigue (P<0.0001, P<0.0001). Bathing did not affect the recovery of physiological muscle fatigue, while the recovery of subjective muscle fatigue was accelerated markedly by 10-min hot water bathing. These findings suggest that utilizing hot water bathing could primarily contribute to the recovery of subjective muscle fatigue. The recovery of physiological muscle fatigue needs more time than the recovery of subjective fatigue feeling.